(1) Field of the Invention
The present invention relates to a silicon nitride sintered body excellent in the flexural strength and oxidation resistance at high temperatures, which is suitably used for a heat engine such as a gas turbine or a turbo rotor, and a process for the preparation thereof.
(2) Description of the Related Art
A silicon nitride sintered body is excellent in strength, hardness and thermochemical stability at high temperatures, and therefore, has attracted attention as engineering ceramics, especially a material of a heat engine.
Since silicon nitride is difficult to sinter, in the production of a silicon nitride sintered body, in general, a sintering aid such as a rare earth element oxide is added, and the hot press method, the normal pressure firing method, the gas pressure firing method and the like are adopted. Recently, there is proposed a method of producing a sintered body having a high density and a high strength by forming an impermeable seal composed of a glass or the like on the surface of a molded body of silicon nitride having a desired composition and firing the molded body under a high pressure (hereinafter referred to as "seal HIP method").
For example, U.S. Pat. No. 4,102,698 to Lange et al. proposes a sintered ceramic composition of the Si.sub.3 N.sub.4 -SiO.sub.2 -Y.sub.2 O.sub.3 system, and U.S. Pat. No. 4,234,343 to Anderson proposes a ceramic composition of the Si.sub.3 N.sub.4 -SiO.sub.2 -M.sub.2 O.sub.3 system in which M represents a combination of a rare earth element with other element such as Al, Cr or Mg. If Al.sub.2 O.sub.3 or MgO is contained in a sintered body, a low-melting-point substance is formed in the grant boundary of the sintered body and the high-temperature strength and the high-temperature oxidation resistance are degraded. Accordingly, from the viewpoint of the high-temperature characteristics of the sintered body, a ceramic composition of the simple ternary Si.sub.3 N.sub.4 -RE.sub.2 O.sub.3 (rare earth element oxide)-SiO.sub.2 system substantially free of the foregoing oxides is now examined.
From the viewpoint of the texture of the sintered body, the grain boundary phase in the sintered body attracts attention as the factor determining the high-temperature characteristics, and the trial is made to substantially crystallize the grain boundary phase for improving the strength of the grain boundary phase per se. Accordingly, there is recently adopted a method in which in a composition of the above-mentioned simple ternary system, the firing conditions are modified or the sintered body is heat-treated so that a crystal phase composed of the Si.sub.3 N.sub.4 -RE.sub.2 O.sub.3 (rare earth element oxide)-SiO.sub.2 system, such as apatite, YAM or wollastonite, is precipitated in the grain boundary.
However, although the crystallization of the grain boundary phase is effective to some extent for improving the high-temperature strength, it is very difficult to precipitate only a specific crystal phase stably in the grain boundary, and it sometimes happens that a low-melting-point glass phase other than the crystal phase is formed in the grain boundary to degrade the characteristics.
For example, if a crystal of the melilite structure is precipitated in the grain boundary phase, although reduction of the high-temperature strength in an inert atmosphere is not caused, in an oxidizing atmosphere which is a practical condition, the grain boundary phase is unstable and the strength is degraded with the change of the volume of the crystal phase. In the case where a crystal of the apatite, wollastonite or YAM structure is precipitated, the stability in an oxidizing atmosphere is slightly improved over the case where a crystal of the melilite structure is precipitated, but a static fatigue is caused during long-period use. As means for overcoming this disadvantage, there is recently proposed a sintered body in which a diopside structure and an apatite structure are precipitated in the grain boundary phase by adding RE.sub.2 O.sub.3 (in which RE represents a rare earth element) and a metal oxide such as MgO to Si.sub.3 N.sub.4 (see Japanese Unexamined Patent Publication No. 62-207765).
According to this technique, degradation of the strength, such as static fatigue, is small even if the sintered body is placed in an oxidizing atmosphere for a long time, but since a metal oxide need to be added in addition to RE.sub.2 O.sub.3, the melting point of the liquid phase necessary for sintering is low and hence, the strength at 1400.degree. C. is drastically degraded.